1. Field of the Invention
The present invention relates to a proportional control pneumatic cylinder serving as an air actuator device for a manually operable hydraulic pump valve having a valve spool moving in a bore in a valve housing. The hydraulic pump valve is a combination of a hydraulic pump and a hydraulic valve associated therewith in a single combined housing unit. The air actuator device is in the form of an actuator housing adapted for attachment on the hydraulic pump valve housing and having a bore carrying a piston coaxially engagable with the end of the hydraulic pump valve spool. The air actuator device is provided with spaced openings in the actuator housing which communicate with the bore therein to introduce air to act on opposite sides of the piston. Factory pretensioned resilient means in the form of a spring is provided on the piston normally urging the piston and spool to a center closed or hold position and providing resistance to movement of the piston and spool in both directions from the center closed position. The surface areas on which the air acts on the opposite sides of the piston are approximately equal so that the movement of the piston is approximately proportional to the movement of the actuating lever in both the raised position and the lowered position of the actuating lever on the pneumatic control valve that supplies air to the opposite sides of the piston. Also, the pneumatic control valve provides control over the majority of the range of travel of the pneumatic control valve's actuating lever instead of just over a small portion of the range of travel of the actuating lever.
2. Description of the Related Art
The teaching found in U.S. Pat. No. 4,585,024 issued to Alexander J. Esseniyi on Apr. 29, 1986 is a good example of the current state of technology relating to the present invention. The Esseniyi invention is an air actuator that attaches to a hydraulic pump valve housing and associated valve spool as a means of controlling the movement of the valve spool. Such arrangements are typically found on dump trucks and similar vehicles as a means of raising and lowering the truck's dump bed. The air actuator is provided with a spring for biasing a pneumatic piston provided in the air actuator and the attached spool to a center closed or hold position where the bed of the truck is neither being raised or lowered.
The air actuator is operated by air supplied by a pneumatic control valve. Air from the pneumatic control valve enters the air actuator device via one of two spaced openings provided in the actuator housing which communicate with the bore therein to introduce air to act on opposite sides of the piston. Introduction of air on one side of the piston forces the piston and the attached valve spool in one direction that causes the hydraulic pump valve to raise the truck bed. Likewise, introduction of air on the other side of the piston forces the piston and the attached valve spool in the opposite direction, thereby causing the hydraulic pump valve to lower the truck bed.
One of the problems encountered with the use of this type of air actuator is that the pneumatic control valve only controls the movement of the spool over a small portion of the range of travel of the pneumatic control valve's actuating lever. The cause of this problem lies in the fact that the surface areas on the opposite sides of the piston on which air acts are not equal or balanced, yet both of the opposite sides are biased by the same spring and are operated by roughly the same truck air pressure. Normal truck air pressure is in the range of 70 to 120 psi. This imbalance on the opposite sides of the piston normally translates into acceptable control over raising the truck bed, but poor control over lowering the truck bed. Thus, because of the unbalanced situation or the unbalanced ratio of the two surface areas located on either side of the piston on which the air acts, the operator has control over the movement of the piston over the majority of the range of travel of the actuating lever when the actuating lever is moving from the hold position to the fully raised position, but has control over the movement of the piston over only a small portion of the range of travel of the actuating lever when the actuating lever is moving from the hold position to the fully lowered position. This means that a small movement by the operator of the actuating lever toward the lowered position will result in a large movement of the piston and attached hydraulic pump valve spool, which results in lowering the truck bed abruptly. It is more critical that the operator have good control over the movement of the truck bed as it is lowered than when it is raised because the control is working against gravity when the truck bed is being raised, but is working, with gravity when the truck bed is being lowered. Thus, if the operator does not have good control over the lowering of the truck bed, the truck bed will fall under the influence of gravity back down onto the truck. Although good control over the movement of the truck bed when it is being raised is desirable, good control over the movement of the truck bed when it is being lowered is essential, particularly when the truck bed is being lowered while still partially loaded, as for example when the truck is being used to lay down asphalt material in the construction of roads.
Although not illustrated in the accompanying drawings, still a further problem with some prior art air actuators is that the various parts of the actuators are shipped to the customer unassembled, including the biasing means which is generally a spring. The user then must assembly the parts for some of the prior art air actuators. Assembly includes compressing the spring to provide tension on the piston and attached spool to keep the spool in the hold position until a minimum amount of air pressure is exerted on the spool and piston. However, it is difficult for the user to assembly the parts because to provide tension on the spring requires the user to push against the spool that can move longitudinally and can also rotate.
The present invention addresses the assembly problem by providing a proportional control pneumatic cylinder that has a factory produced piston assembly containing a factory pretensioned spring which makes for easy installation of the device on the hydraulic pump valve by the user.
Also, the present invention addresses the unbalanced situation by providing surfaces on which the air acts on opposite sides of the piston which are approximately equal in surface area. This allows the invention to approximate proportionally controlled movement of the spool of a hydraulic pump valve that raises and lowers the truck bed with the movement of the actuating lever of the pneumatic control valve both when raising and lowering the truck bed and provides control over movement of the spool through the majority of the range of travel of the pneumatic control valve's actuating lever from the hold position to the fully raised position and from the hold position to the fully lowered position.